178 DR. JAMES BOTTOMLEY ON COLORIMETRY. 



with the degree of oxidation ; also it seems to be colouring- 

 matter in suspension rather than in solution. I had some 

 hopes of succeeding by mixing solutions of nickel and cobalt 

 salts. On reference to the ' Philosophical Magazine/ 

 vol. vi. p. 1 5:, I find that the colour-relations of nickel and 

 cobalt had been studied by Mr. Thomas Bayley with a view 

 to the quantitative determination of these metals founded 

 upon the complementary character of their colours. He 

 states^ " The fact will have been observed by chemists that 

 solutions of nickel and cobalt salts are so far complemen- 

 tary in colour that when they are mixed together the re- 

 sulting liquid, if moderately dilute, is hardly to be distin- 

 guished from pure water .^' After considering the nature 

 of the absorption-spectra of nickel and cobalt salts, he 

 states, "li the spectra were exactly complementary, on 

 superimposing the nickel spectrum upon the cobalt spec- 

 trum the dark part on the one would cover exactly the 

 light part on the other. This, however, though nearly the 



case, is not exactly so this is why the solution obtained 



by mixing strong solutions of nickel and cobalt is not grey, 

 but reddish brown in colour." Some experiments which 

 I made seemed to confirm the opinion of Mr. Bayley. The 

 nickel solutions contained 0*05 grm. of NiSO^. per cub. 

 c; and the cobalt solution contained 0*05 grm. C0SO4 per 

 cub. c. A mixture consisting of 50 cub. c. of cobalt so- 

 lution with 100 cub. c. of nickel solution, contained in a 

 white porcelain basin, seemed to be a grey tinted with 

 pink in the shallower parts, and having the tendency to 

 pass into a yellowish tint as the depth increased. I now 

 poured the fluid into a tall glass cylinder covered externally 

 with black cloth except a circular aperture of 8 millim. 

 diameter at the bottom. When I looked through the 

 column of fluid at a white surface, the colour was decided, 

 resembling somewhat the pigment known as yellow ochre. 



